1. Field of the Invention
The present invention relates to imaging modules using solid state sensors for recording a target image, including optical code readers and digital cameras. Aspects of the invention are particularly useful in linear sensor-based and two-dimensional sensor-based, handheld bar code readers. More specifically, the present invention relates to reduced form factor modules which can be utilized in portable or mobile computers or consumer appliances.
2. Description of the Related Art
Optical codes are patterns made up of image areas having different light reflective or light absorptive properties, which are typically assembled in accordance with a priori rules. The term “bar code” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which bar code scanners are one type. Optical code readers are used in both fixed and portable installations in many diverse environments such as in stores for checkout services, in manufacturing locations for work flow and inventory control and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target bar code from a printed listing of many bar codes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor which is manually directed at a target code.
Most conventional code readers are designed to read one-dimensional bar code symbols. The bar code is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light reflecting characteristics. One example of a one-dimensional bar code is the UPC/EAN code.
Bar codes can be read employing solid state imaging devices. For example, an image sensor may be employed which has a two-dimensional array of cells or photo sensors which correspond to image elements or pixels in a field of view of the device. Such an image sensor may be a two-dimensional or area charge coupled device (CCD) and associated circuits for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view. A one-dimensional linear array of photodiodes may also be used in detecting a bar code reflection image (see, e.g., U.S. Pat. No. 6,138,915 to Danielson et al., which is herein expressly incorporated by reference).
It is known in the art to use a CCD image sensor and objective lens assembly in an optical code reader. In the past, such systems have employed complex objective lens assemblies originally designed for relatively expensive video imaging systems. Such systems may have a single sharp focus and a limited depth of field, which along with conventional aiming, illumination and signal processing and decoding algorithms, limits the versatility and working range of the system.
Other conventional imaging systems are designed primarily for reading optical codes. Such reading systems involve the assembly and alignment of several small parts. These parts may include a lens, an aperture and a 2D image sensor array such as a CCD chip. Such a structure is illustrated, for example, in U.S. patent application Ser. No. 09/096,578 for Imaging Engine and Method for Code Readers to Correa et al. filed Jun. 12, 1998 and assigned to Symbols Technologies, Inc, which is the same assignee as the present application. The '578 application is hereby incorporated by reference herein. A miniature imager adapted for use in a hand mounted code reader is also disclosed in U.S. patent application Ser. No. 09/684,514 filed Oct. 10, 2000 to Patel et al., which is also assigned to the same assignee as the present application, and is hereby incorporated by reference. Other systems are described at U.S. Pat. Nos. 5,814,803, 6,042,012, and 6,311,895 (describing a CMOS imager).
The design of an imaging system is dependent upon the size of the package in which the imaging system is to be manufactured. Conventional imaging systems which utilize off-the-shelf components are difficult to miniaturize due to the limited selection of off-the-shelf components. Further, due to various optical phenomena in the design of the system, various tradeoffs between a component size and the quality of a scanned image must be weighed in the selection of components. Additionally, the selection of certain components for an imager may, due to optical phenomena, limit the choice of other components for the miniature imager. It is therefore desired to have a miniaturized scanner with an optimal selection of components which provides an adequate scanned image, while minimizing the physical size and shape, i.e., the form factor, of the system.
To provide illumination and to assist in aiming, imaging systems can employ either lasers or light emitting diodes (LEDs). LEDs may be preferred over lasers since the incoherent nature of the LED light source does not produce the speckle noise impact that is produced by lasers. Further, LEDs are more cost effective than lasers due to the ease of manufacturing and packaging of LEDs. Additionally, LEDs can be built more compactly and are easier to surface mount than lasers. However, compared to lasers, LEDs are not an ideal point source. Specifically, light produced by an LED is less focused which produces an increased line thickness of the projected light. To reduce the line thickness of the light produced by an LED, many designers place a mechanical slit in front of the LED. However, the mechanical slit reduces the amount of light that is projected by the LED onto an object. Accordingly, it is desired to provide an LED-based aiming beam generation system that has a reduced line thickness of the projected light without severely reducing the amount of light projected by the LED.
In designing a digital image data capture device it is desirable, particularly in mobile products, to have the ability to capture a good quality digital photo and also read barcodes. To read barcodes with the same camera used for taking digital pictures typically results in neither system being optimized. Digital photos are better taken with a system focused at infinity with a relatively large aperture for collecting light. Barcode readers have better performance when the focus is pulled in typically between 4″ to 8″ from the principal plain. This has to do with maintaining the spatial sampling required for successfully decoding the barcode to be acquired. Although a large aperture is also desired for collecting light in barcode reading it is rarely acceptable due to the depth of field requirement typically referred to as the barcode reader's working range. The larger the aperture, the larger the optical blur circle and the quicker the image becomes out of focus as it is moved from the in focus position.
Several methods for solving these conflicting requirements have been previously proposed (see, e.g., U.S. Pat. Nos. 6,208,812, 5,821,523, 5,548,359, 5,530,498, and 5,525,788, which are hereby incorporated by reference). Some of these arrangements utilize moving optical elements to change the focus. The most common scheme is the auto-focus mechanism in a typical camera. More specific embodiments related to the field of barcode involve the sliding/moving of a piece of glass that ultimately changes the focus position of the optical system between two distinct settings. This moving of the piece of glass is much cheaper than a continuous auto-focus system but it still requires moving parts and the overhead of assembly complexity, reliability, and repeatability. These moving part schemes do have a distinct advantage in that they change the focus position for the entire field of view based on the desired image capture application (i.e., photo vs. barcode reading).
It has been shown that the retail and consumer marketplaces do not require the entire field of view to be changed between two focus positions. Retail and consumer applications for digital cameras, particularly in mobile devices, have the general demand for systems that take good quality digital pictures and read 1-D barcodes, and there is little demand for systems that read 2D barcodes in subsets of these market segments, particularly within the consumer market. By involving this related requirement of reading only 1-D barcodes well and taking good quality digital pictures, embodiments are provided herein that do not utilize moving parts, thus reducing cost and complexity, and improving reliability.
As mentioned, it is also desired to have bar code scanning equipment with extended depth of focus. U.S. Pat. No. 5,210,398, e.g., provides an advantageous system including a complex lens having multiple focal lengths. U.S. Pat. Nos. 5,640,001 and 6,138,915 describe scanners having focus control for operation over a range of distances. U.S. Pat. No. 5,814,803 describes several embodiments for extending the depth of focus including using gradient index lenses, staggered optical flats and primary collecting lenses followed by staggered secondary lenses, among others. U.S. Pat. No. 6,382,513 describes a system including a segmented collection mirror. Advantageous embodiments including simplified optics with fixed focus positions are provided herein below.